1. Field of the Invention
2. Discussion of the Background
The present invention relates to a guide vane for an axial fan.
When a gas passes through a fan the gas is deflected by the impeller rotor blades and a pressure increase is obtained across the impeller. However, this deflection means that the gas flow velocity has a rotational component after passage through the impeller. This rotational component forms rotational energy which is often lost in the continued gas transport downstream the fan.
It is known to arrange a ring of guide vanes downstream of the impeller to make use of this rotation energy and then raise the pressure increase of the fan as well as its efficiency. The rotation energy of the gas flow after the impeller is thus converted into a static pressure increase on passing over the guide vanes. This conversion is not free of losses, and to minimize the losses it is essential that the inlet angle of the guide vanes substantially coincides with the direction of gas flow leaving the impeller. If the inlet side of the guide vanes is not adapted to the direction of the impinging gas, a strong release of the flow is obtained at the guide vane, with large energy losses and an acompanying decrease of the fan efficiency as a result. The guide vanes are also implemented so that the gas on the outlet side is given a substantially axial directional component.
It has been found that the magnitude of the rotational component varies in the radial direction, which means that the angle which the flow direction forms with the central axis varies with the radius. The flow is very complex, and secondary effects result in the fact that the rotational component after the fan blades will be larger at the root and top of the blades. At the root of blades, i.e. at the point of attachment of the blades to the hub, the gas flow is given an increased rotational component by back flow in gaps and by the rotation of the hub, and at the top of the blades there is an increased rotation as a result of back flow which lowers the axial component. In addition, it should be noted that the exterior limiting surface, e.g. the wall of a flow duct or the like, not only retards the tangential movement component but also the axial one. Taken together this gives the unexpected radial variation of the flow direction illustrated in FIG. 1.
FIG. 1 thus illustrates the result of measurements made with an axial fan. As will be seen, the flow direction angle towards the central axis is greater at the top and root of the blade, and the angle passes through a minimum value therebetween. The exact appearance of the graph is affected by such parameters as the blade angles on the impeller and the selected operating point in the corresponding fan diagram (pressure flow diagram), but the shape of the graph is qualitatively the same, with a minimum between the positions of the blade root and top.
In attempts to adapt the inlet angle of the guide vane to the rotation component, which varies radially, guide vanes have been produced with varying curvature, which requires a very complicated manufacturing technique, however.
Guide vanes have also been made with an oblique edge between the inner and outer longitudinal edges of the guide vane, so that the arcuate length of the guide vane along the inner edge is longer than the arcuate length along the radially outer edge. For a constant curvature of the guide vane there is thus obtained a greater inlet angle at the radially inward portion of the guide vane than at its radially outward portion.